Reduced metal content ceramic composite bodies

ABSTRACT

This invention relates generally to a novel method for removing metal from a formed self-supporting body. A self-supporting body is made by reactively infiltrating a molten parent metal into a bed or mass containing a boron donor material and a carbon donor material (e.g., boron carbide) and/or a boron donor material and a nitrogen donor material (e.g., boron nitride) and, optionally, one or more inert fillers. Once the self-supporting body is formed, it is then subjected to appropriate conditions which causes metallic constituent contained in the self-supporting body to be at least partially removed.

The present application in a U.S. National Stage Application ofInternational Patent Application Serial No. US/91/04950, filed on Jul.12, 1992, as a continuation-in-part of U.S. patent application Ser. No.07/551,288filed on Jul. 12, 1990and now abandoned.

1. Technical Field

This invention relates generally to a novel method for removing metalfrom a formed self-supporting body. A self-supporting body is made byreactively infiltrating a molten parent metal into a bed or masscontaining a boron source material and a carbon source material (e.g.,boron carbide) and/or a boron source material and a nitrogen sourcematerial (e.g., boron nitride) and, optionally, one or more inertfillers. Once the self-supporting body is formed, it is then subjectedto appropriate conditions which causes metallic constituent contained inthe self-supporting body to be at least partially removed.

2. Background Art

In recent years, there has been an increasing interest in the use ofceramics for structural applications historically served by metals. Theimpetus for this interest has been the superiority of ceramics withrespect to certain properties, such as corrosion resistance, hardness,wear resistance, modulus of elasticity, and refractory capabilities whencompared with metals.

However, a major limitation on the use of ceramics for such purposes isthe feasibility and cost of producing the desired ceramic structures.For example, the production of ceramic boride bodies by the methods ofhot pressing, reaction sintering and reaction hot pressing is wellknown. In the case of hot pressing, fine powder particles of the desiredboride are compacted at high temperatures and pressures. Reaction hotpressing involves, for example, compacting at elevated temperatures andpressures boron or a metal boride with a suitable metal-containingpowder. U.S. Pat. No. 3,937,619 to Clougherty describes the preparationof a boride body by hot pressing a mixture of powdered metal with apowdered diboride, and U.S. Pat. No. 4,512,946 to Brun describes hotpressing ceramic powder with boron and a metal hydride to form a boridecomposite.

However, these hot pressing methods require special handling andexpensive special equipment, they are limited as to the size and shapeof the ceramic part produced, and they typically involve low processproductivities and high manufacturing cost.

A second major limitation on the use of ceramics for structuralapplications is their general lack of toughness (i.e. damage toleranceor resistance to fracture). This characteristic tends to result insudden, easily induced, catastrophic failure of ceramics in applicationsinvolving even rather moderate tensile stresses. This lack of toughnesstends to be particularly common in monolithic ceramic boride bodies.

One approach to overcome this problem has been to attempt to useceramics in combination with metals, for example, as cermets or metalmatrix composites. The objective of this approach is to obtain acombination of the best properties of the ceramic (e.g. hardness and/orstiffness) and the metal (e.g. ductility). U.S. Pat. No. 4,585,618 toFresnel, et al., discloses a method of producing a cermet whereby a bulkreaction mixture of particulate reactants, which react to produce asintered self-sustaining ceramic body, is reacted while in contact witha molten metal. The molten metal infiltrates at least a portion of theresulting ceramic body. Exemplary of such a reaction mixture is onecontaining titanium, aluminum and boron oxide (all in particulate form),which is heated while in contact with a pool of molten aluminum. Thereaction mixture reacts to form titanium diboride and alumina as theceramic phase, which is infiltrated by the molten aluminum. Thus, thismethod uses the aluminum in the reaction mixture principally as areducing agent. Further, the external pool of molten aluminum is notbeing used as a source of precursor metal for a boride forming reaction,but rather it is being utilized as a means to fill the pores in theresulting ceramic structure. This creates cermets which are wettable andresistant to molten aluminum. These cermets are particularly useful inaluminum production cells as components which contact the moltenaluminum produced but preferably remain out of contact with the moltencryolite.

European Application 0,113,249 to Reeve, et al. discloses a method formaking a cermet by first forming in situ dispersed particles of aceramic phase in a molten metal phase, and then maintaining this moltencondition for a time sufficient to effect formation of an intergrownceramic network. Formation of the ceramic phase is illustrated byreacting a titanium salt with a boron salt in a molten metal such asaluminum. A ceramic boride is developed in situ and becomes anintergrown network. There is, however, no infiltration, and further theboride is formed as a precipitate in the molten metal. Both examples inthe application expressly state that no grains were formed of TiAl₃,AlB₂, or AlB₁₂, but rather TiB₂ is formed demonstrating the fact thatthe aluminum is not the metal precursor to the boride.

U.S. Pat. No. 3,864,154 to Gazza, et al. discloses a ceramic-metalsystem produced by infiltration. An AlB₁₂ compact was impregnated withmolten aluminum under vacuum to yield a system of these components.Other materials prepared included SiB₆ --Al; B--Al; B₄ C--Al/Si; andAlB₁₂ --B--Al. There is no suggestion whatsoever of a reaction, and nosuggestion of making composites involving a reaction with theinfiltrating metal nor of any reaction product embedding an inert filleror being part of a composite.

U.S. Pat. No. 4,605,440 to Halverson, et al., discloses that in order toobtain B₄ C--Al composites, a B₄ C--Al compact (formed by cold pressinga homogeneous mixture of B₄ C and Al powders) is subjected to sinteringin either a vacuum or an argon atmosphere. There is no infiltration ofmolten metal from a pool or body of molten precursor metal into apreform. Further, there is no mention of a reaction product embedding aninert filler in order to obtain composites utilizing the favorableproperties of the filler.

While these concepts for producing cermet materials have in some casesproduced promising results, there is a general need for more effectiveand economical methods to prepare boride-containing materials.

DESCRIPTION OF COMMONLY OWNED PATENTS AND PATENT APPLICATIONS

Many of the above-discussed problems associated with the production ofboride-containing materials have been addressed in U.S. Pat. No.4,885,130 (hereinafter "Patent '130"), which issued on Dec. 5, 1989, inthe names of T. Dennis Claar, Steven M. Mason, Kevin P. Pochopien, DannyR. White, and William B. Johnson, and is entitled "Process for PreparingSelf-Supporting Bodies and Products Produced Thereby".

Briefly summarizing the disclosure of Patent '130, self-supportingceramic bodies are produced by utilizing a parent metal infiltration andreaction process (i.e., reactive infiltration) in the presence of a masscomprising boron carbide. Particularly, a bed or mass comprising boroncarbide and, optionally, one or more of a boron donor material and acarbon donor material, is infiltrated by molten parent metal, and thebed may be comprised entirely of boron carbide or only partially ofboron carbide, thus resulting in a self-supporting body comprising, atleast in part, one or more parent metal boron-containing compounds,which compounds include a parent metal boride or a parent metal borocarbide, or both, and typically also may include a parent metal carbide.It is also disclosed that the mass comprising boron carbide which is tobe infiltrated may also contain one or more inert fillers mixed with theboron carbide. Accordingly, by combining an inert filler, the resultwill be a composite body having a matrix produced by the reactiveinfiltration of the parent metal, said matrix comprising at least oneboron-containing compound, and the matrix may also include a parentmetal carbide, the matrix embedding the inert filler. It is furthernoted that the final composite body product in either of theabove-discussed embodiments (i.e., filler or no filler) may include aresidual metal as at least one metallic constituent of the originalparent metal.

Broadly, in the disclosed method of Patent '130, a mass comprising boroncarbide and, optionally, one or more of a boron donor material and acarbon donor material, is placed adjacent to or in contact with a bodyof molten metal or metal alloy, which is melted in a substantially inertenvironment within a particular temperature envelope. The molten metalinfiltrates the mass comprising boron carbide and reacts with at leastthe boron carbide to form at least one reaction product. The boroncarbide (and/or the boron donor material and/or the carbon donormaterial) is reducible, at least in part, by the molten parent metal,thereby forming the parent metal boron-containing compound (e.g., aparent metal boride and/or boro compound under the temperatureconditions of the process). Typically, a parent metal carbide is alsoproduced, and in certain cases, a parent metal boro carbide is produced.At least a portion of the reaction product is maintained in contact withthe metal, and molten metal is drawn or transported toward the unreactedmass comprising boron carbide by a wicking or a capillary action. Thistransported metal forms additional parent metal boride, carbide, and/orboro carbide and the formation or development of a ceramic body iscontinued until either the parent metal or mass comprising boron carbidehas been consumed, or until the reaction temperature is altered to beoutside of the reaction temperature envelope. The resulting structurecomprises one or more of a parent metal boride, a parent metal borocompound, a parent metal carbide, a metal (which, as discussed in Patent'130, is intended to include alloys and intermetallics), or voids, orany combination thereof. Moreover, these several phases may or may notbe interconnected in one or more dimensions throughout the body. Thefinal volume fractions of the boron-containing compounds (i.e., borideand boron compounds), carbon-containing compounds, and metallic phases,and the degree of interconnectivity, can be controlled by changing oneor more conditions, such as the initial density of the mass comprisingboron carbide, the relative amounts of boron carbide and parent metal,alloys of the parent metal, dilution of the boron carbide with a filler,the amount of boron donor material and/or carbon donor material mixedwith the mass comprising boron carbide, temperature, and time.Preferably, conversion of the boron carbide to the parent metal boride,parent metal boro compound(s) and parent metal carbide is at least about50%, and most preferably at least about 90%.

The typical environment or atmosphere which was utilized in Patent '130was one which is relatively inert or unreactive under the processconditions. Particularly, it was disclosed that an argon gas, or avacuum, for example, would be suitable process atmospheres. Stillfurther, it was disclosed that when zirconium was used as the parentmetal, the resulting composite comprised zirconium diboride, zirconiumcarbide, and residual zirconium metal. It was also disclosed that whenaluminum parent metal was used with the process, the result was analuminum boro carbide such as Al₃ B₄₈ C₂, AlB₁₂ C₂ and/or AlB₂₄ C₄, withaluminum parent metal and other unreacted unoxidized constituents of theparent metal remaining. Other parent metals which were disclosed asbeing suitable for use with the processing conditions included silicon,titanium, hafnium, lanthanum, iron, calcium, vanadium, niobium,magnesium, and beryllium.

Still further, it is disclosed that by adding a carbon donor material(e.g., graphite powder or carbon black) and/or a boron donor material(e.g., a boron powder, silicon borides, nickel borides and iron borides)to the mass comprising boron carbide, the ratio of parentmetal-boride/parent metal-carbide can be adjusted. For example, ifzirconium is used as the parent metal, the ratio of ZrB₂ /ZrC can bereduced if a carbon donor material is utilized (i.e., more ZrC isproduced due to the addition of a carbon donor material in the mass ofboron carbide) while if a boron donor material is utilized, the ratio ofZrB₂ /ZrC can be increased (i.e., more ZrB₂ is produced due to theaddition of a boron donor material in the mass of boron carbide). Stillfurther, the relative size of ZrB₂ platelets which are formed in thebody may be larger than platelets that are formed by a similar processwithout the use of a boron donor material. Thus, the addition of acarbon donor material and/or a boron donor material may also affect themorphology of the resultant material.

In another related Patent, specifically, U.S. Pat. No. 4,915,736(hereinafter referred to as "Patent '736"), issued in the names of TerryDennis Claar and Gerhard Hans Schiroky, on Apr. 10, 1990, and entitled"A Method of Modifying Ceramic Composite Bodies By a CarburizationProcess and Articles Made Thereby", additional modification techniquesare disclosed. Specifically, Patent '736 discloses that a ceramiccomposite body made in accordance with the teachings of, for example,Patent '130 can be modified by exposing the composite to a gaseouscarburizing species. Such a gaseous carburizing species can be producedby, for example, embedding the composite body in a graphitic bedding andreacting at least a portion of the graphitic bedding with moisture oroxygen in a controlled atmosphere furnace. However, the furnaceatmosphere should comprise typically, primarily, a non-reactive gas suchas argon. It is not clear whether impurities present in the argon gassupply the necessary 0₂ for forming a carburizing species, or whetherthe argon gas merely serves as a vehicle which contains impuritiesgenerated by some type of volatilization of components in the graphiticbedding or in the composite body. In addition, a gaseouscarburizing-species could be introduced directly into a controlledatmosphere furnace during heating of the composite body.

Once the gaseous carburizing species has been introduced into thecontrolled atmosphere furnace, the setup should be designed in such amanner to permit the carburizing species to be able to contact at leasta portion of the surface of the composite body buried in the looselypacked graphitic powder. It is believed that carbon in the carburizingspecies, or carbon from the graphitic bedding, will dissolve into theinterconnected zirconium carbide phase, which can then transport thedissolved carbon throughout substantially all of the composite body, ifdesired, by a vacancy diffusion process. Moreover, Patent '736 disclosesthat by controlling the time, the exposure of the composite body to thecarburizing species and/or the temperature at which the carburizationprocess occurs, a carburized zone or layer can be formed on the surfaceof the composite body. Such process could result in a hard,wear-resistant surface surrounding a core of composite material having ahigher metal content and higher fracture toughness.

Thus, if a composite body was formed having a residual parent metalphase in the amount of between about 5-30 volume percent, such compositebody could be modified by a post-carburization treatment to result infrom about 0 to about 2 volume percent, typically about 1/2 to about 2volume percent, of parent metal remaining in the composite body.

U.S. Pat. No. 4,885,131 (hereinafter "Patent '131"), issued in the nameof Marc S. Newkirk on Dec. 5, 1989, and entitled "Process For PreparingSelf-Supporting Bodies and Products Produced Thereby", disclosesadditional reactive infiltration formation techniques. Specifically,Patent '131 discloses that self-supporting bodies can be produced by areactive infiltration of a parent metal into a mixture of a bed or masscomprising a boron donor material and a carbon donor material. Therelative amounts of reactants and process conditions may be altered orcontrolled to yield a body containing varying volume percents ofceramic, metals, ratios of one ceramic or another and porosity.

In another related patent application, specifically, copending U.S.patent application Ser. No. 07/296,770 (hereinafter referred to as"Application '770"), filed in the names of Terry Dennis Claar et al., onJan. 13, 1989, and entitled "A Method of Producing Ceramic CompositeBodies", additional reactive infiltration formation techniques aredisclosed. Specifically, application '770 discloses various techniquesfor shaping a bed or mass comprising boron carbide into a predeterminedshape and thereafter reactively infiltrating the bed or mass comprisingboron carbide to form a self-supporting body of a desired size andshape.

U.S. Pat. No. 5,011,063, which issued on Apr. 30, 1991, from U.S. patentapplication Ser. No. 07/560,491, filed Jul. 23, 1990, which is acontinuation of U.S. patent application Ser. No. 07/296,837 (hereinafterreferred to as "Patent '063"), filed in the name of Terry Dennis Claaron Jan. 13, 1989, and entitled "A Method of Bonding A Ceramic CompositeBody to a Second Body and Articles Produced Thereby", discloses variousbonding techniques for bonding self-supporting bodies to secondmaterials. Particularly, this patent discloses that a bed or masscomprising one or more boron-containing compounds is reactivelyinfiltrated by a molten parent metal to produce a self-supporting body.Moreover, residual or excess metal is permitted to remain bonded to theformed self-supporting body. The excess metal is utilized to form a bondbetween the formed self-supporting body and another body (e.g., a metalbody or a ceramic body of any particular size or shape).

The reactive infiltration of a parent metal into a bed or masscomprising boron nitride is disclosed in U.S. Pat. No. 4,904,446(hereinafter "Patent "446"), issued in the names of Danny Ray White etal., on Feb. 27, 1990, and entitled "Process for PreparingSelf-Supporting Bodies and Products Made Thereby". Specifically, thispatent discloses that a bed or mass comprising boron nitride can bereactively infiltrated by a parent metal. A relative amount of reactantsand process conditions may be altered or controlled to yield a bodycontaining varying volume percents of ceramic, metal and/or porosity.Additionally, the self-supporting body which results comprises aboron-containing compound, a nitrogen-containing compound and,optionally, a metal. Additionally, inert fillers may be included in theformed self-supporting body.

A further post-treatment process for modifying the properties ofproduced ceramic composite bodies is disclosed in U.S. Pat. No.5,004,714 (hereinafter "Patent 714"), which issued on Apr. 2, 1991, fromU.S. patent application Ser. No. 07/296,966, filed in the names of TerryDennis Claar et al., on Jan. 13, 1989, and entitled "A Method ofModifying Ceramic Composite Bodies By Post-Treatment Process andArticles Produced Thereby". Specifically, Patent '714 discloses thatself-supporting bodies produced by a reactive infiltration technique canbe post-treated by exposing the formed bodies to one or more metals andheating the exposed bodies to modify at least one property of thepreviously formed composite body. One specific example of apost-treatment modification step includes exposing a formed body to asiliconizing environment.

U.S. Pat. No. 5,019,539 hereinafter "Patent '539"), which issued on May28, 1991, from U.S. patent application Ser. No. 07/296,961, filed in thenames of Terry Dennis Claar et al., on Jan. 13, 1989, and entitled "AProcess for Preparing Self-Supporting Bodies Having Controlled Porosityand Graded Properties and Products Produced Thereby", discloses reactinga mixture of powdered parent metal with a bed or mass comprising boroncarbide and, optionally, one or more inert fillers. Additionally, it isdisclosed that both a powdered parent metal and a body or pool of moltenparent metal can be induced to react with a bed or mass comprising boroncarbide. The body which is produced is a body which has controlled orgraded properties.

The disclosures of each of the above-discussed Commonly Owned U.S.patent applications and Patents are herein expressly incorporated byreference.

SUMMARY OF THE INVENTION

In accordance with a first step of the present invention,self-supporting ceramic bodies are produced by utilizing a parent metalinfiltration and reaction process (i.e. reactive infiltration) in thepresence of a bed or mass comprising a boron donor and a carbon donorand/or a boron donor and a nitrogen donor, such as, for example, boroncarbide or boron nitride. Such bed or mass is infiltrated by moltenparent metal, and the bed may be comprised entirely of boron carbide,boron nitride, and/or mixtures of boron donor materials, carbon donormaterials and nitrogen donor materials. Depending on the particularreactants involved in the reactive infiltration, the resulting bodieswhich are produced comprise one or more reaction products such as one ormore parent metal boron-containing compounds, and/or one or more parentmetal carbon-containing compounds and/or one or more parent metalnitrogen-containing compounds, etc. Alternatively, the mass to beinfiltrated may contain one or more inert fillers admixed therewith toproduce a composite by reactive infiltration, which composite comprisesa matrix of one or more of the aforementioned reaction products and alsomay include residual unreacted or unoxidized constituents of the parentmetal. The filler material may be embedded by the formed matrix. Thefinal product may include a metallic constituent comprising one or moremetallic components of the parent metal. Still further, in some cases itmay be desirable to add a carbon donor material (i.e., acarbon-containing compound) and/or a boron donor material (i.e., aboron-containing compound) and/or a nitrogen donor material (i.e., anitrogen-containing compound) to the bed or mass which is to beinfiltrated to modify, for example, the relative amounts of one formedreaction product to another, thereby modifying resultant mechanicalproperties of the composite body. Still further, the reactantconcentrations and process conditions may be altered or controlled toyield a body containing varying volume percents of ceramic compounds,metal and/or porosity.

Broadly, in accordance with the first step of the method according tothis invention, the bed or mass which is to be reactively infiltratedmay be placed adjacent to or in contact with a body of molten metal ormetal alloy, which is melted in a substantially inert environment withina particular temperature envelope. Appropriate parent metals for use inthe present invention include such metals as zirconium, titanium,hafnium, aluminum, vanadium, chromium, niobium, etc., and particularlypreferred parent metals include zirconium, titanium and hafnium. Themolten metal infiltrates the mass and reacts with at least oneconstituent of the bed or mass to be infiltrated to form one or morereaction products. At least a portion of the formed reaction product ismaintained in contact with the metal, and molten metal is drawn ortransported toward the remaining unreacted mass by a wicking orcapillary action. This transported metal forms additional reactionproduct upon contact with the remaining unreacted mass, and theformation or development of a ceramic body is continued until the parentmetal or remaining unreacted mass has been consumed, or until thereaction temperature is altered to be outside the reaction temperatureenvelope. The resulting structure comprises, depending upon theparticular materials comprising the bed or mass which is to bereactively infiltrated, one or more of a parent metal boride, a parentmetal boro compound, a parent metal carbide, a parent metal nitride, ametal (which as used herein is intended to include alloys andintermetallics), or voids, or a combination thereof, and these severalphases may or may not be interconnected in one or more dimensions. Thefinal volume fractions of the reaction products and metallic phases, andthe degree of interconnectivity, can be controlled by changing one ormore conditions, such as the initial density of the mass to bereactively infiltrated, the relative amounts and chemical composition ofthe materials contained within the mass which is to be reactivelyinfiltrated, the amount of parent metal provided for reaction, thecomposition of the parent metal, the presence and amount of one or morefiller materials, temperature, time, etc.

Typically, the mass to be reactively infiltrated should be at leastsomewhat porous so as to allow for wicking the parent metal through thereaction product. Wicking occurs apparently either because any volumechange on reaction does not fully close off pores through which parentmetal can continue to wick, or because the reaction product remainspermeable to the molten metal due to such factors as surface energyconsiderations which render at least some of its grain boundariespermeable to the parent metal.

In another aspect of the first step of the invention, a composite isproduced by the transport of molten parent metal into the bed or masswhich is to be reactively infiltrated which has admixed therewith one ormore inert filler materials. In this embodiment, one or more suitablefiller materials are mixed with the bed or mass to be reactivelyinfiltrated. The resulting self-supporting ceramic-metal composite thatis produced typically comprises a dense microstructure which comprises afiller embedded by a matrix comprising at least one parent metalreaction product, and also may include a substantial quantity of metal.Typically, only a small amount of material (e.g., a small amount ofboron carbide) is required to promote the reactive infiltration process.Thus, the resulting matrix can vary in content from one composedprimarily of metallic constituents thereby exhibiting certain propertiescharacteristics of the parent metal; to cases where a high concentrationof reaction product is formed, which dominates the properties of thematrix. The filler may serve to enhance the properties of the composite,lower the raw materials cost of the composite, or moderate the kineticsof the reaction product formation reactions and the associated rate ofheat evolution. The precise starting amounts and composition ofmaterials utilized in the reactive infiltration process can be selectedso as to result in a desirable body which is compatible with the secondstep of the invention.

In another aspect of the first step of the present invention, thematerial to be reactively infiltrated is shaped into a preformcorresponding to the geometry of the desired final composite. Reactiveinfiltration of the preform by the molten parent metal results in acomposite having the net shape or near net shape of the preform, therebyminimizing expensive final machining and finishing operations. Moreover,to assist in reducing the amount of final machining and finishingoperations, a barrier material can at least partially, or substantiallycompletely, surround the preform. For example, a graphite material(e.g., a graphite mold, a graphite tape product, a graphite coating,etc.) is particularly useful as a barrier for such parent metals aszirconium, titanium, or hafnium, when used in combination with preformsmade of, for example, boron carbide, boron nitride, boron and carbon.Still further, by placing an appropriate number of through-holes havinga particular size and shape in the aforementioned graphite mold, theamount of porosity which typically occurs within a composite bodymanufactured according to the first step of the present invention can bereduced. Typically, a plurality of holes is placed in a bottom portionof the mold, or that portion of the mold toward which reactiveinfiltration occurs. The holes function as a venting means which permitthe removal of, for example, argon gas which has been trapped in thepreform as the parent metal reactive infiltration front infiltrates thepreform.

Still further, the procedures discussed above herein in the Section"Discussion of Commonly Owned U.S. Patents No. and Patent Applications"may be applicable in connection with the first step of the presentinvention.

Once a self-supporting body has been formed in accordance with the firststep of the present invention, then the second step of the presentinvention is put into effect. The second step of the present inventioninvolves subjecting at least a portion of the formed self-supportingbody to appropriate processing conditions which causes at least aportion of the metallic constituent to be at least partially removedfrom the self-supporting body.

In a first embodiment of the invention, a metallic constituent of aself-supporting composite body produced in accordance with the firststep of the present invention can be at least partially, orsubstantially completely, removed by causing the metallic constituent toreact with an adjacent (e.g., permeable) mass of material. To achieveremoval of the metallic constituent, at least a portion of the permeablemass is placed into contact with at least a portion of the metallicconstituent contained within the self-supporting body. Thus, at least aportion of the metallic constituent should be at least partiallyaccessible, or should be made to be at least partially accessible, fromat least one surface of the self-supporting composite body.

The amount or selected portion of metallic constituent which is causedto be removed from the self-supporting body can be controlled to achievea desirable metal content. Specifically, substantially all metallicconstituent located in a certain area within a self-supporting compositebody (e.g., located near a surface of the self-supporting compositebody) may be substantially completely removed from that selected area,thereby leaving other areas of metallic constituent within the compositebody substantially undisturbed. Moreover, if the metallic constituent issubstantially interconnected throughout the composite body,substantially all the metallic constituent could be removed. Thevolumetric amount of metallic constituent to be removed from theself-supporting composite body depends upon the ultimate application forthe composite body. Thus, the present invention may be utilized merelyas a surface modification process, or it could be used to removesubstantially all the metallic constituent from a self-supportingcomposite body.

In a preferred embodiment of the second step of the present invention,the self-supporting body may be substantially completely surrounded byand contacted with an appropriate mass of material. In this embodiment,at least a portion of, or substantially all of, the metallic constituentcould be removed from substantially all surfaces of the self-supportingbody, so long as the metallic constituent is at least partiallyaccessible, or can be made to be at least partially accessible, fromsuch surfaces.

In another preferred embodiment of the second step of the presentinvention, only a portion of the self-supporting body may be contactedwith the appropriate mass of material. In this preferred embodiment, themetallic constituent could be selectively removed from that surfacewhich is in contact with the permeable mass. In this preferredembodiment, it is possible to achieve a grading of properties within aself-supporting body from one side of the body relative to another sideof the body. Such grading could permit the self-supporting body to beused for a number of different applications.

A number of materials may be placed into contact with self-supportingbodies formed in accordance with the first step of the presentinvention. Acceptable materials include carbide, nitrides, borides, etc.A primary selection criteria for the material comprising the permeablemass is that the permeable mass should be at least partially wettable bythe metallic component of the self-supporting body. Moreover, thepermeable mass can be selected so that it is substantially nonreactivewith or very reactive with the metallic component of the self-supportingbody. In the case where the permeable mass is selected so that it issubstantially nonreactive with the metallic constituent comprising aself-supporting body, very little conversion of metallic constituent toanother phase can be expected; whereas if a metallic constituent isreactive with a material in the permeable mass, at least partialconversion of the metallic constituent to another material can beexpected.

As stated above, the amount of metallic constituent that is removed froma self-supporting body can be controlled to be within any particulardesirable range. For example, if a self-supporting body was formed tocontain about 20 volume percent metallic constituent, substantially allof the metallic constituent could be removed by following the teachingsof the present invention. Additionally, it has been observed that whenthe material comprising the permeable mass is substantially nonreactive(e.g., chemically) with metallic constituent contained in theself-supporting body, substantially no conversion of metallicconstituent to another material occurs. Rather, substantially completeremoval of the metallic constituent from the self-supporting body isessentially all that occurs. This fact has been proven by quantitativeimage analysis.

In another embodiment of the second step of the present invention, ametallic constituent of a self-supporting composite body produced inaccordance with the first step of the present invention can be at leastpartially, or substantially completely removed by subjecting theself-supporting composite body to an appropriate treatment (e.g.,thermal etching, chemical etching, vacuum etching, etc.) to remove atleast a portion of the metallic constituent from the self-supportingbody. This embodiment for removing a metallic constituent from aself-supporting ceramic body may be used alone, or in combination withthe metal removal techniques discussed above.

A first treatment for removing metallic constituent from aself-supporting composite body is to place the self-supporting compositebody in a substantially inert bed that is contained within a crucible orother refractory container. The container and its contents are thenplaced into a furnace having an inert atmosphere (e.g., argon or anyother non-reactive gas) and heated to temperatures where the metallicconstituent will, preferably have a sufficiently high vapor pressure.This temperature or preferred temperature range can vary depending uponsuch factors as the composition of the parent metal, the time ofheating, the end composition of the metallic constituent in theself-supporting composite body, as well as any potential damage that mayoccur to other constituents in the self-supporting body. At a suitabletemperature, metallic constituent may vaporize from at least a portionof the self-supporting composite body. By maintaining thesetemperatures, the metallic constituent may continue to vaporize and becarried away from the composite body as by, for example, a suitableventing means within the furnace. Alternatively, rather than providingan inert atmosphere, a vacuum may be provided which enhances the removalof at least a portion of the metallic constituent under appropriateprocessing conditions.

A second treatment for removing metallic constituent from aself-supporting composite body is to contact or immerse theself-supporting composite body in a suitable leachant which may, forexample, dissolve or disperse out at least a portion of the metallicconstituent. The leachant may be any liquid or gas (e.g., an acidicmaterial, caustic material or reactive material), the composition ofwhich will depend upon such factors as the composition of metal, thetime of contact or immersion, etc. The time of contact or immersion ofthe self-supporting composite body in the leachant will depend upon theamount and type of metal component, and where the metallic constituentis situated with respect to the surface(s). The more metallicconstituent that is contained is in the self-supporting body, the longerit may take for such metal to be leached or etched out. This extractiontechnique may be facilitated by heating of the leachant or, in the caseof a liquid leachant, by agitating the bath of leachant.

Definitions

As used in this specification and the appended claims, the terms beloware defined as follows:

"Parent metal" refers to that metal, e.g. zirconium, titanium, hafnium,etc., which is the precursor to the polycrystalline oxidation reactionproduct, that is, the parent metal boride, parent metal carbide, parentmetal nitride, or other parent metal compound, and includes that metalas a pure or relatively pure metal, a commercially available metalhaving impurities and/or alloying constituents therein, and an alloy inwhich that metal precursor is the major constituent; and when a specificmetal is mentioned as the parent metal, e.g. zirconium, titanium,hafnium, etc., the metal identified should be read with this definitionin mind unless indicated otherwise by the context.

"Parent metal boride" and "parent metal boro compounds" mean a reactionproduct containing boron formed upon reaction between a boron donormaterial, such as boron carbide or boron nitride, and the parent metaland includes a binary compound of boron with the parent metal as well asternary or higher order compounds.

"Parent metal nitride" means a reaction product containing nitrogenformed upon reaction of a nitrogen donor material, such as boron nitrideand the parent metal.

"Parent metal carbide" means a reaction product containing carbon formedupon reaction of a carbon donor material, such as boron carbide, and theparent metal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the setup used to fabricate a plateletreinforced composite body;

FIG. 2 is a schematic view of the setup used to remove the residualmetal from the platelet reinforced composite body; and

FIG. 3 is an approximately 100× magnification photomicrograph of across-section of the platelet reinforced composite body after the metalremoval process.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

In accordance with a first step of the present invention,self-supporting ceramic bodies are produced by utilizing a parent metalinfiltration and reaction process (i.e. reactive infiltration) in thepresence of a bed or mass comprising a boron donor and a carbon donorand/or a boron donor and a nitrogen donor, such as, for example, boroncarbide or boron nitride. Such bed or mass is infiltrated by moltenparent metal, and the bed may be comprised entirely of boron carbide,boron nitride, and/or mixtures of boron donor materials, carbon donormaterials and nitrogen donor materials. Depending on the particularreactants involved in the reactive infiltration, the resulting bodieswhich are produced comprise one or more reaction products such as one ormore parent metal boron-containing compounds, and/or one or more parentmetal carbon-containing compounds and/or one or more parent metalnitrogen-containing compounds, etc. Alternatively, the mass to beinfiltrated may contain one or more inert fillers admixed therewith toproduce a composite by reactive infiltration, which composite comprisesa matrix of one or more of the aforementioned reaction products and alsomay include residual unreacted or unoxidized constituents of the parentmetal. The filler material may be embedded by the formed matrix. Thefinal product may include a metallic constituent comprising one or moremetallic constituents of the parent metal. Still further, in some casesit may be desirable to add a carbon donor material (i.e., acarbon-containing compound) and/or a boron donor material (i.e., aboron-containing compound) and/or a nitrogen donor material (i.e., anitrogen-containing compound) to the bed or mass which is to beinfiltrated to modify, for example, the relative amounts of one formedreaction product to another, thereby modifying resultant mechanicalproperties of the composite body. Still further, the reactantconcentrations and process conditions may be altered or controlled toyield a body containing varying volume percents of ceramic compounds,metal and/or porosity.

Broadly, in accordance with the first step of the method according tothis invention, the bed or mass which is to be reactively infiltratedmay be placed adjacent to or in contact with a body of molten metal ormetal alloy, which is melted in a substantially inert environment withina particular temperature envelope. Appropriate parent metals for use inthe present invention include such metals as zirconium, titanium,hafnium, aluminum, vanadium, chromium, niobium, etc., and particularlypreferred parent metals include zirconium, titanium and hafnium. Themolten metal infiltrates the mass and reacts with at least oneconstituent of the bed or mass to be infiltrated to form one or morereaction products. At least a portion of the formed reaction product ismaintained in contact with the metal, and molten metal is drawn ortransported toward the remaining unreacted mass by a wicking orcapillary action. This transported metal forms additional reactionproduct upon contact with the remaining unreacted mass, and theformation or development of a ceramic body is continued until the parentmetal or remaining unreacted mass has been consumed, or until thereaction temperature is altered to be outside the reaction temperatureenvelope. The resulting structure comprises, depending upon theparticular materials comprising the bed or mass which is to bereactively infiltrated, one or more of a parent metal boride, a parentmetal boro compound, a parent metal carbide, a parent metal nitride, ametal (which as used herein is intended to include alloys andintermetallics), or voids, or a combination thereof, and these severalphases may or may not be interconnected in one or more dimensions. Thefinal volume fractions of the reaction products and metallic phases, andthe degree of interconnectivity, can be controlled by changing one ormore conditions, such as the initial density of the mass to bereactively infiltrated, the relative amounts and chemical composition ofthe materials contained within the mass which is to be reactivelyinfiltrated, the amount of parent metal provided for reaction, thecomposition of the parent metal, the presence and amount of one or morefiller materials, temperature, time, etc.

Typically, the mass to be reactively infiltrated should be at leastsomewhat porous so as to allow for wicking the parent metal through thereaction product. Wicking occurs apparently either because any volumechange on reaction does not fully close off pores through which parentmetal can continue to wick, or because the reaction product remainspermeable to the molten metal due to such factors as surface energyconsiderations which render at least some of its grain boundariespermeable to the parent metal.

In another aspect of the first step of the invention, a composite isproduced by the transport of molten parent metal into the bed or masswhich is to be reactively infiltrated which has admixed therewith one ormore inert filler materials. In this embodiment, one or more suitablefiller materials are mixed with the bed or mass to be reactivelyinfiltrated. The resulting self-supporting ceramic-metal composite thatis produced typically comprises a dense microstructure which comprises afiller embedded by a matrix comprising at least one parent metalreaction product, and also may include a substantial quantity of metal.Typically, only a small amount of material (e.g., a small amount ofboron carbide) is required to promote the reactive infiltration process.Thus, the resulting matrix can vary in content from one composedprimarily of metallic constituents thereby exhibiting certain propertiescharacteristics of the parent metal; to cases where a high concentrationof reaction product is formed, which dominates the properties of thematrix. The filler may serve to enhance the properties of the composite,lower the raw materials cost of the composite, or moderate the kineticsof the reaction product formation reactions and the associated rate ofheat evolution. The precise starting amounts and composition ofmaterials utilized in the reactive infiltration process can be selectedso as to result in a desirable body which is compatible with the secondstep of the invention.

In another aspect of the first step of the present invention, thematerial to be reactively infiltrated is shaped into a preformcorresponding to the geometry of the desired final composite. Reactiveinfiltration of the preform by the molten parent metal results in acomposite having the net shape or near net shape of the preform, therebyminimizing expensive final machining and finishing operations. Moreover,to assist in reducing the amount of final machining and finishingoperations, a barrier material can at least partially, or substantiallycompletely, surround the preform. For example, a graphite material(e.g., a graphite mold, a graphite tape product, a graphite coating,etc.) is particularly useful as a barrier for such parent metals aszirconium, titanium, or hafnium, when used in combination with preformsmade of, for example, boron carbide, boron nitride, boron and carbon.Still further, by placing an appropriate number of through-holes havinga particular size and shape in the aforementioned graphite mold, theamount of porosity which typically occurs within a composite bodymanufactured according to the first step of the present invention can bereduced. Typically, a plurality of holes is placed in a bottom portionof the mold, or that portion of the mold toward which reactiveinfiltration occurs. The holes function as a venting means which permitthe removal of, for example, argon gas which has been trapped in thepreform as the parent metal reactive infiltration front infiltrates thepreform.

Still further, the procedures discussed above herein in the Section"Discussion of Commonly Owned U.S. Patents and Patent Applications" maybe applicable in connection with the first step of the presentinvention.

Once a self-supporting body has been formed in accordance with the firststep of the present invention, then the second step of the presentinvention is put into effect. The second step of the present inventioninvolves subjecting at least a portion of-the formed self-supportingbody to appropriate processing conditions which causes at least aportion of the metallic constituent to be at least partially removedfrom the self-supporting body.

In a first embodiment of the invention, a metallic constituent of aself-supporting composite body produced in accordance with the firststep of the present invention can be at least partially, orsubstantially completely, removed by causing the metallic constituent toreact with an adjacent (e.g., permeable) mass of material. To achieveremoval of the metallic constituent, at least a portion of the permeablemass is placed into contact with at least a portion of the metallicconstituent contained within the self-supporting body. Thus, at least aportion of the metallic constituent should be at least partiallyaccessible, or should be made to be at least partially accessible, fromat least one surface of the self-supporting composite body.

The amount or selected portion of metallic constituent which is causedto be removed from the self-supporting body can be controlled to achievea desirable metal content. Specifically, substantially all metallicconstituent located in a certain area within a self-supporting compositebody (e.g., located near a surface of the self-supporting compositebody) may be substantially completely removed from that selected area,thereby leaving other areas of metallic constituent within the compositebody substantially undisturbed. Moreover, if the metallic constituent issubstantially interconnected throughout the composite body,substantially all the metallic constituent could be removed. Thevolumetric amount of metallic constituent to be removed from theself-supporting composite body depends upon the ultimate application forthe composite body. Thus, the present invention may be utilized merelyas a surface modification process, or it could be used to removesubstantially all the metallic constituent from a self-supportingcomposite body.

In a preferred embodiment of the second step of the present invention,the self-supporting body may be substantially completely surrounded byand contacted with an appropriate mass of material. In this embodiment,at least a portion of, or substantially all of, the metallic constituentcould be removed from substantially all surfaces of the self-supportingbody, so long as the metallic constituent is at least partiallyaccessible, or can be made to be at least partially accessible, fromsuch surfaces.

In another preferred embodiment of the second step of the presentinvention, only a portion of the self-supporting body may be contactedwith the appropriate mass of material. In this preferred embodiment, themetallic constituent could be selectively removed from that surfacewhich is in contact with the permeable mass. In this preferredembodiment, it is possible to achieve a grading of properties within aself-supporting body from one side of the body relative to another sideof the body. Such grading could permit the self-supporting body to beused for a number of different applications.

A number of materials may be placed into contact with self-supportingbodies formed in accordance with the first step of the presentinvention. Acceptable materials include carbide, nitrides, borides, etc.A primary selection criteria for the material comprising the permeablemass is that the permeable mass should be at least partially wettable bythe metallic component of the self-supporting body. Moreover, thepermeable mass can be selected so that it is substantially nonreactivewith or very reactive with the metallic component of the self-supportingbody. In the case where the permeable mass is selected so that it issubstantially nonreactive with the metallic constituent comprising aself-supporting body, very little conversion of metallic constituent toanother phase can be expected; whereas if a metallic constituent isreactive with a material in the permeable mass, at least partialconversion of the metallic constituent to another material can beexpected.

As stated above, the amount of metallic constituent that is removed froma self-supporting body can be controlled to be within any particulardesirable range. For example, if a self-supporting body was formed tocontain about 20 volume percent metallic constituent, substantially allof the metallic constituent could be removed by following the teachingsof the present invention. Additionally, it has been observed that whenthe material comprising the permeable mass is substantially nonreactive(e.g., chemically) with metallic constituent contained in theself-supporting body, substantially no conversion of metallicconstituent to another material occurs. Rather, substantially completeremoval of the metallic constituent from the self-supporting body isessentially all that occurs. This fact has been proven by quantitativeimage analysis.

In another embodiment of the second step of the present invention, ametallic constituent of a self-supporting composite body produced inaccordance with the first step of the present invention can be at leastpartially, or substantially completely removed by subjecting theself-supporting composite body to an appropriate treatment (e.g.,thermal etching, chemical etching, vacuum etching, etc.) to remove atleast a portion of the metallic constituent from the self-supportingbody. This embodiment for removing a metallic constituent from aself-supporting ceramic body may be used alone, or in combination withthe metal removal techniques discussed above.

A first treatment for removing metallic constituent from aself-supporting composite body is to place the self-supporting compositebody in a substantially inert bed that is contained within a crucible orother refractory container. The container and its contents are thenplaced into a furnace having an inert atmosphere (e.g., argon or anyother non-reactive gas) and heated to temperatures where the metallicconstituent will, preferably have a sufficiently high vapor pressure.This temperature or preferred temperature range can vary depending uponsuch factors as the composition of the parent metal, the time ofheating, the end composition of the metallic constituent in theself-supporting composite body, as well as any potential damage that mayoccur to other constituents in the self-supporting body. At a suitabletemperature, metallic constituent may vaporize from at least a portionof the self-supporting composite body. By maintaining thesetemperatures, the metallic constituent may continue to vaporize and becarried away from the composite body as by, for example, a suitableventing means within the furnace. Alternatively, rather than providingan inert atmosphere, a vacuum may be provided which enhances the removalof at least a portion of the metallic constituent under appropriateprocessing conditions.

A second treatment for removing metallic constituent from aself-supporting composite body is to contact or immerse theself-supporting composite body in a suitable leachant which may, forexample, dissolve or disperse out at least a portion of the metallicconstituent. The leachant may be any liquid or gas (e.g., an acidicmaterial, caustic material or reactive material), the composition ofwhich will depend upon such factors as the composition of metal, thetime of contact or immersion, etc. The time of contact or immersion ofthe self-supporting composite body in the leachant will depend upon theamount and type of metal component, and where the metallic constituentis situated with respect to the surface(s). The more metallicconstituent that is contained is in the self-supporting body, the longerit may take for such metal to be leached or etched out. This extractiontechnique may be facilitated by heating of the leachant or, in the caseof a liquid leachant, by agitating the bath of leachant.

The following are examples of the present invention. The Examples areintended to be illustrative of various aspects of the present invention,however, these examples should not be construed as limiting the scope ofthe invention.

EXAMPLE 1

This Example demonstrates a technique for removing the residual metallicconstituent from a platelet reinforced composite body. A lay-up used toform the platelet reinforced composite body is shown in FIG. 1. Thelay-up used to remove the residual metallic constituent from the formedplatelet reinforced composite body is shown in FIG. 2.

About 600 grams of methylene chloride (J. T. Baker, Inc., Phillipsburg,N.J.) was poured into an approximately 1/2× gallon (2 liter) NALGENE®jar (Nalge Company, Rochester, N.Y.). About 4 grams of XUS 40303.00Experimental Binder (Dow Chemical Company, Midland, Mich.) was added tothe methylene chloride and allowed to dissolve. About 400 grams of 1000grit TETRABOR® boron carbide particulate (ESK Engineering Material, NewCanaan, Conn.) having an average particle size of about 5 microns wasstirred into the solution of binder and methylene chloride to form aslurry.

As shown in FIG. 1, a grade ATJ graphite mold 10 (Union CarbideCorporation, Carbon Products Division, Cleveland, Ohio) having innerdimensions measuring about 3.0 inches (76 mm) square and about 4.0inches (102 mm) high, was filled with methylene chloride (J. T. BakerInc., Phillipsburg, N.J.) and placed into a drying box at substantiallyroom temperature for about one hour to allow the methylene chloridesolvent to saturate the graphite mold 10. After soaking for about anhour, the residual methylene chloride was poured out, and a quantity ofthe slurry containing the boron carbide particulate was sediment castinto the saturated graphite mold 10. The graphite mold 10 containing thesediment cast boron carbide preform 12 were placed back into the dryingbox and allowed to dry overnight.

The graphite mold 10 and the dried sediment cast preform 12 were thenfired in a resistance heated controlled atmosphere furnace to remove thebinder from the preform. Specifically, the graphite mold 10 and itscontents was placed into the furnace chamber, which was then evacuatedto about 30 inches (762 mm) of mercury vacuum, and then backfilled withargon gas. After repeating this evacuation and backfilling procedure, anargon gas flow rate of about two liters per minute at an overpressure ofabout 1 psi (7 kPa) was established. The furnace temperature was thenincreased from substantially room temperature to a temperature of about250° C. at a rate of about 44° C. per hour. Upon reaching a temperatureof about 250° C., the temperature was then increased to about 300° C. ata rate of about 50° C. per hour. Upon reaching a temperature of about300° C., the temperature was then increased to about 400° C. at areduced rate of about 10° C. per hour. Upon reaching a temperature ofabout 400° C., the temperature was then increased to about 600° C. at arate of about 50° C. per hour. After maintaining a temperature of about600° C. for about four hours, the binder had been substantiallycompletely removed from the sediment cast preform, and the furnace wasthen cooled to substantially room temperature at a rate of about 200° C.per hour. After cooling to substantially room temperature, the graphitemold 10 and sediment cast preform 12 were removed from the furnace. Theweight of the preform 12 itself was found to be about 80 grams and thepreform thickness was calculated to about 0.46 inch (12 mm). The bulkdensity of the sediment cast boron carbide preform 12 was calculated toabout 1.18 grams per cubic centimeter, corresponding to a theoreticaldensity of about 46.8%.

About 534.6 grams of zirconium sponge 14, (Consolidated Astronautics,Inc., location) was poured on top of the sediment cast boron carbidepreform 12 in the graphite mold 10, and the zirconium was levelled toform a lay-up.

The lay-up comprising the graphite mold 10 and its contents were placedinto a vacuum furnace. The furnace chamber was evacuated to about 30inches (762 mm) of mercury vacuum and then with argon gas. Afterrepeating this evacuation and backfilling procedure, an argon gas flowrate of about two liters per minute was established through the furnaceat an overpressure of about 2 psi (14 kPa). The furnace temperature wasthen increased from about room temperature to a temperature of about1900° C. at a rate of about 375° C. per hour. After maintaining atemperature of about 1900° C. for about two hours, reactive infiltrationof the molten zirconium metal into the boron carbide preform wassubstantially complete. Accordingly, the furnace temperature wasdecreased to substantially room temperature at a rate of about 900° C.per hour. After cooling to substantially room temperature, the graphitemold 10 and its contents were removed from the furnace. The plateletreinforced composite body formed by the reactive infiltration of thezirconium metal into the sediment cast boron carbide preform 12 weighedabout 601 grams and measured about 3.0 inches (76 mm) square by about0.73 inch (18.5 mm) thick. The formed body comprised zirconium diboride,zirconium carbide, and a metallic constituent comprising residualzirconium alloy.

A portion of the formed platelet reinforced composite was sectionedusing electro-discharge machining, mounted in a thermoplastic polymerand polished using diamond polishing compound in preparation forexamination by optical microscopy. Quantitative image analysis of thepolished sample showed a residual metal content of about 16.3%.

Referring to FIG. 2, a coupon 20 of the platelet reinforced compositebody was machined from the formed 3.0 inches (76 mm) square tile usingelectro-discharge machining. The machined coupon 20 weighed about 2.33grams and measured about 21.6 mm long by about 16.4 mm wide by about 1.1mm thick.

Zirconium carbide particulate 22 (-325 mesh, Atlantic EquipmentEngineers, Bergenfield, N.J.) having substantially all particles smallerthan about 45 microns in diameter was poured into a grade ATJ graphitecrucible 24 (Union Carbide Corporation, Carbon Products Division,Cleveland, Ohio) measuring in its interior about 2.0 inches (51 mm)square by about 3.0 inches (76 mm) high to a depth of about 1.0 inch (25mm). The machined coupon 20 of the platelet reinforced compositematerial was placed flat onto the levelled surface of the zirconiumcarbide particulate material 22. Additional zirconium carbideparticulate 22 was added to the graphite crucible 24 to substantiallycompletely cover the coupon 20 of platelet reinforced composite materialuntil a total depth of zirconium particulate 22 of about 2.0 inches (51mm) was achieved.

The graphite crucible 24 and its contents were then placed into aresistance heated controlled atmosphere furnace. The furnace chamber wasevacuated to about 30 inches (762 mm) of mercury vacuum and thenbackfilled with argon gas. An argon gas flow rate of about two litersper minute was established through the furnace at an overpressure ofabout 2 psi (14 kPa). The furnace temperature was then increased fromsubstantially room temperature to a temperature of about 1800° C. at arate of about 400° C. per hour. After maintaining a temperature of about1800° C. for about one hour, the temperature was then decreased at arate of about 350° C. per hour. After the temperature had been reducedto substantially room temperature, the graphite crucible 24 and itscontents were removed from the furnace and disassembled.

The coupon 20 of the platelet reinforced composite body was recoveredand mounted and polished for examination in the optical microscope insubstantially the same manner as was the originally formed plateletreinforced composite body. FIG. 3 is an approximately 100× magnificationphotomicrograph showing a portion of the composite coupon. Quantitativeimage analysis of the underlying microstructure reported only about0.26% residual metal and about 0.35% porosity.

The volume fraction ratio of zirconium diboride to zirconium carbidewithin the underlying microstructure as determined also by quantitativeimage analysis, was recorded before and after the second heating toremove the metallic constituent from the body. The ratio was found to besubstantially unchanged, indicating that the residual metal in theplatelet reinforced composite had not carburized upon heating in azirconium carbide environment. Moreover, the almost completedisappearance of residual metal from the original platelet reinforcedcomposite body may be attributed to physical removal of such metal fromthe body. That substantially no porosity was seen in the plateletreinforced composite body following the metal removal process indicatesthat some shrinkage or sintering of the body may have occurred.Specifically, it appeared that the body decreased in volume by about16%.

This Example thus illustrates a technique for removing the residualmetal from a platelet reinforced composite body wherein relative amountsof the zirconium diboride and zirconium carbide phases are leftsubstantially unchanged.

We claim:
 1. A method for removing metal from a self-supporting body,said self-supporting body being made by:providing at least one firstself-supporting body which is made by a process comprising (i) heating aparent metal in a substantially inert atmosphere to a temperature aboveits melting point to form a body of molten parent metal; (ii) contactingsaid body of molten parent metal with a first permeable mass which is tobe reactively infiltrated; (iii) maintaining said temperature for a timesufficient to permit infiltration of molten parent metal into said firstpermeable mass which is to be reactively infiltrated and to permitreaction of said molten parent metal with said first permeable mass toform at least one boron-containing compound; (iv) continuing saidinfiltration reaction for a time sufficient to produce at least onefirst self-supporting body containing at least some metallicconstituent, said metal removing method comprising the steps of:contacting at least a portion of said at least one first self-supportingbody with a second permeable mass which is capable of removing at leasta portion of said metallic constituent; heating said at least one firstself-supporting body and said permeable mass to cause metallicconstituent from said first self-supporting body to be at leastpartially removed from said first self-supporting body into said secondpermeable mass without substantial reaction of said metallicconstituent; and continuing said removing of said metallic constituentfor a time sufficient to remove a desired amount of said metallicconstituent.
 2. A method for removing metal from a self-supporting body,said self-supporting body being made by:providing at least one firstself-supporting body which is made by a process comprising (i) heating aparent metal in a substantially inert atmosphere to a temperature aboveits melting point to form a body of molten parent metal; (ii) contactingsaid body of molten parent metal with a first permeable mass which is tobe reactively infiltrated; (iii) maintaining said temperature for a timesufficient to permit infiltration of molten parent metal into said firstpermeable mass which is to be reactively infiltrated and to permitreaction of said molten parent metal with said first permeable mass toform at least one boron-containing compound; (iv) continuing saidinfiltration reaction for a time sufficient to produce at least onefirst self-supporting body containing at least some metallicconstituent, said metal removing method comprising the steps of:providing a source of vacuum to at least a portion of saidself-supporting body; and heating at least said portion of saidself-supporting body to at least a temperature sufficient for saidvacuum source to enhance removal of a desired amount of said metallicconstituent.
 3. The method of claim 1, wherein said parent metalcomprises at least one metal selected from the group consisting oftitanium, zirconium, hafnium, aluminum, vanadium, chromium and niobium,and alloys thereof.
 4. The method of claim 1, wherein said at least onefirst self-supporting body comprises at least one material selected fromthe group consisting of one or more boron-containing materials, one ormore carbon-containing materials, one or more nitrogen-containingmaterials and metal.
 5. The method of claim 1, wherein said firstpermeable mass comprises at least one material selected from the groupconsisting of a boron donor material and a carbon donor material, and aboron donor material and a nitrogen donor material, and mixturesthereof.
 6. The method of claim 5, wherein said first permeable masscomprises at least one material selected from the group consisting ofboron, carbon, boron carbide and boron nitride.
 7. The method of claim1, wherein said at least one first body comprises at least one materialselected from the group consisting of a parent metal boride, a parentmetal boro compound, a parent metal carbide, a parent metal nitride,residual metal and voids.
 8. The method of claims 1, wherein said firstpermeable mass comprises a preform.
 9. The method of claims 1, furthercomprising providing a barrier in contact with at least one surface ofsaid first permeable mass.
 10. The method of claim 1, wherein saidremoving is carried out for a time sufficient to remove at least aportion of said metallic constituent from at least one surface of saidfirst self-supporting body or for a time sufficient to substantiallycompletely remove said metallic constituent from said firstself-supporting body.
 11. The method of claim 2, wherein said secondpermeable mass is at least partially wettable by said metallicconstituent.
 12. The method of claim 2, wherein said second permeablemass comprises at least one material selected from the group consistingof carbides, borides and nitrides.
 13. The method of claim 2, whereinsaid second permeable mass comprises zirconium carbide.
 14. The methodof claim 1, wherein said metallic constituent is removed from at least aportion of said first self-supporting body, thereby resulting in aself-supporting body comprising a graded metallic constituent.
 15. Themethod of claim 1, wherein said parent metal comprises at least onemetal selected from the group consisting of zirconium, titanium andhafnium, said first permeable mass comprises at least one materialselected from the group consisting of boron, carbon, boron carbide andboron nitride, and said metallic constituent is substantially completelyremoved from said self-supporting body.
 16. The method of claim 2,wherein said parent metal comprises zirconium, said first permeable masscomprises boron carbide and said second permeable mass compriseszirconium carbide.
 17. A method for removing at least a portion of atleast one metallic constituent contained within a multi-phase compositebody, comprising:contacting at least a portion of a surface of saidmultiphase composite body with a permeable mass capable of removing atleast one molten metallic constituent from said multiphase compositebody; heating said multi-phase body to at least the melting point ofsaid at least one metallic constituent; and without substantial reactionof said at least one metallic constituent infiltrating at least aportion of the permeable mass with said at least one metallicconstituent, thereby reducing the amount of metallic constituent in themulti-phase composite body.
 18. The method of claim 17, wherein said atleast one metallic constituent of the composite body is selectivelyremoved from only a portion of the composite body.
 19. The method ofclaim 17, wherein substantially all of said at least one metallicconstituent is removed.
 20. The method of claim 17, wherein saidpermeable mass substantially completely surrounds said composite body.21. The method of claim 17, wherein said permeable mass comprises aceramic particulate.